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Monkeys can use basic logic to decipher the order of items in a list

Keeping track of rankings could be useful for monitoring social pecking orders in the wild

KEEPING ORDER Rhesus macaque monkeys don’t need rewards to learn and remember the order of items in a list, a mental feat that may prove handy in the wild.


Monkeys can keep strings of information in order by using a simple kind of logical thought. Rhesus macaque monkeys learned the order of items in a list with repeated exposure to pairs of items plucked from the list, say psychologist Greg Jensen of Columbia University and colleagues. The animals drew basic logical conclusions about pairs of listed items, akin to assuming that if A comes before B and B comes before C, then A comes before C, the scientists conclude July 30 in Science Advances.

Importantly, rewards given to monkeys didn’t provide reliable guidance to the animals about whether they had correctly ordered pairs of items. Monkeys instead worked out the approximate order of images in the list, and used that knowledge to make choices in experiments about which of two images from the list followed the other, Jensen’s group says. Previous studies have suggested that a variety of animals, including monkeys, apes, pigeons, rats and crows, can discern the order of a list of items (SN: 7/5/08, p. 13). But debate persists about whether nonhuman creatures do so only with the prodding of rewards for correct responses or, at least sometimes, by consulting internal knowledge acquired about particular lists.

Jensen’s group designed experimental sessions in which four monkeys completed as many as 600 trials to determine the order of seven images in a list. Images included a hot air balloon, an ear of corn and a zebra. Monkeys couldn’t rely on rewards to guide their choices. In some sessions, animals usually received a larger reward for correctly identifying which of two images came later in the list and a smaller reward for an incorrect response. In other sessions, incorrect responses usually yielded a larger reward than correct responses. Rewards consisted of larger or smaller gulps of water delivered through tubes to the moderately thirsty primates. Monkeys consistently learned list orders in both reward conditions, making relatively few errors by the end of the sessions. Giving rewards for correct responses produced slightly faster list learning, the team found.

Jensen’s study adds to evidence suggesting that, like humans, monkeys can mentally link together pairs of items into lists that guide later choices, says psychologist Regina Paxton Gazes of Bucknell University in Lewisburg, Pa. That’s probably a valuable ability in the wild, she says, because many animals need to monitor where group mates stand in the social pecking order. “An ability to construct, retain, manipulate and reference ordered information may be an evolutionarily ancient, efficient [mental] mechanism for keeping track of relationships between individuals,” she says.

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